1. Field of the Invention
The present invention relates to a zoom lens barrel which includes a rotatable cam ring driven to rotate.
2. Description of the Related Art
In conventional zoom lens barrels, a moving mechanism, provided with a rotatable cam ring driven to rotate and having cam grooves, for linearly moving at least one linearly-guided member (generally a lens support frame), which is positioned radially inside the rotatable cam ring, in an optical axis direction by a rotational force of the rotatable cam ring has been widely adopted. The linearly-guided member is supported so as to be prevented from rotating in order to be capable of moving only linearly in the optical axis direction, and includes cam followers which are respectively engaged in the aforementioned cam grooves of the rotatable cam ring. In the case of such a mechanism having the simplest structure, two sets of cam grooves for moving two lens groups are formed on an inner peripheral surface of the rotatable cam ring while two sets of cam followers on two lens support frames of the two lens groups are engaged in the two sets of cam grooves, respectively, to thereby make it possible to give each of the two lens groups a predetermined zooming path.
The rotatable cam ring used to be rotated simply at a fixed position (i.e., without moving in the optical axis direction), however, in recently-produced zoom lens barrels including a rotatable cam ring, a structure in which the rotatable cam ring is engaged with a stationary ring via helicoids (helicoidal threads) and is rotated while being moved forward and rearward in the optical axis direction has been widely adopted to reduce the length of the zoom lens barrel when retracted. This type of zoom lens barrel is disclosed in Japanese Unexamined Patent Publication 2003-185907.
Nevertheless, in the conventional structure in which the rotatable cam ring rotates at a fixed position, if the length of the rotatable cam ring in the optical axis direction is short, the angle of inclination of each cam groove on the rotatable cam ring with respect to a circumferential direction thereof is great, which causes the rotatable cam ring to require a greater rotational force for rotating the rotatable cam ring. In addition, even in the conventional structure in which the rotatable cam ring moves forward and rearward in the optical axis direction while rotating via helicoids, the inclination of each cam groove on the rotatable cam ring with respect to a circumferential direction thereof is also great. Accordingly, such conventional structures, in which the inclination of each cam groove on the rotatable cam ring with respect to a circumferential direction thereof is great, narrows the degree of design flexibility of the zoom lens barrel and unnecessarily requires the rotatable cam ring to be long in length and have a long movement range.